The art of surveying involves the determination of unknown positions, surfaces or volumes of objects or setting out of known coordinates using angle and distance measurements taken from one or more positions. In order to perform these measurements, frequently a surveying device is used comprising a distance measuring instrument with an integrated distance and angular measurement of the type commonly referred to as a total station or theodolite, i.e. comprising a combination of electronics and optics. A total station is furthermore provided with a computer or processing or control unit with writable information for measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. A more detailed description of such a total station can for example be found in WO 2004/057269 by the same applicant.
With reference to FIG. 1, a measuring or surveying instrument of the type referred to as a total station or theodolite generally includes a movable unit 20 including optical equipment indicated with a lens 30, for example a camera for capturing a field of view and an identified target point within the field of view. The movable unit 20 is rotatably mounted in a housing 40 in such manner that it is pivotable relatively to the housing 40 around a first axis 50 as indicated by double arrow 60. The first axis 50 may also be referred to as the trunnion axis. The housing 40 is rotatably mounted in a base 80 such that the housing 40 is rotatable relatively to the base 80 around a second axis 90 as indicated by double arrow 100. The housing may also be referred to as the alidade portion 40. Thus, by rotating the movable unit 20 around the two axes 50 and 90 the movable unit 40 can be oriented in any desired position for the purpose of carrying out an intended surveying operation. When performing distance measuring or surveying tasks using a distance measuring total station, for example at a work site, a naval work site, a construction work site or a mining work site, a high degree of accuracy is generally required, with acceptable tolerances normally being in the order of arc-seconds for angles and millimeters for distance. The trunnion axis 50 is in an ideal case perpendicular to the second axis 90. Furthermore, the second axis 90 is in an ideal case vertical.
It is desirable that the rotatable mounting of the movable unit 20 and the housing 40 in the housing 40 and the base 80, respectively, comprises an accurate bearing in order to facilitate achieving a high degree of accuracy in measurements such as distance measuring or surveying tasks as described above. For example for the rotation of the housing 40 relatively to the base 80 such a bearing may comprise a V-bearing, i.e. a friction bearing having two defined contact points.
Other examples of bearings are rolling-element bearings, i.e. bearings carrying a load by means of round elements located between two parts of the bearing. An example of rolling-element bearings is radial ball bearings, i.e. bearings having inner and outer races, or lanes, and a set of balls configured in a row which revolves around the ball path. The radial ball bearing may be stressed, or preloaded, radially and/or axially. Each race is a ring with a groove where the balls rest. The groove is usually shaped so that each ball has a slightly loose fit in the groove. Thus, in principle each ball contacts each race at a single point.
However, in high-accuracy applications such as described above rolling-element bearings in general give rise to errors in measurements of distances and/or angles due to imperfections in the rolling-element bearings, i.e. deviations in the geometrical configuration of the rolling-element bearing from the ideal case, which imperfections may give rise to deviation of the first 50 and/or second axis 90 from the respective ‘true’ (reference) axis.